Magnetic disks and disk drives are conventionally employed for storing data in magnetizable form. Typically, one or more disks are rotated on a central axis in combination with data transducing heads positioned in close proximity to the recording surfaces of the disks and moved generally radially with respect thereto. Magnetic disks are usually housed in a magnetic disk unit in a stationary state with a magnetic head having a specific load elastically in contact with and pressed against the surface of the disk. It is extremely difficult to produce a magnetic recording medium for ultra-high density recording having suitable magnetic properties, such as high coercivity, e.g., greater than 2500 Oersteads, and a high overwrite, e.g., about 40 dB, while at the same time exhibiting suitable mechanical properties for read-write performance, such as a small glide height avalanche, e.g., about 0.75 to about 0.85 .mu.m.
In operation, the magnetic disk is normally driven by the contact start stop (CSS) method, wherein the head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by the air flow generated between the sliding surface of the head and the disk. During reading and recording operations, the transducing head is maintained at a controlled distance from the recording surface, supported on a bearing of air as the disk rotates. The magnetic head unit is arranged such that the head can be freely moved in both the circumferential and radial directions of the disk in this floating state allowing data to be recorded on and retrieved from the surface of the disk at a desired position.
Upon terminating operation of the disk drive, the rotational speed of the disk decreases and the head begins to slide against the surface of the disk again and eventually stops in contact with and pressing against the disk. Thus, the transducing head contacts the recording surface whenever the disk is stationary, accelerated from a stop and during deceleration just prior to completely stopping. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic operation consisting of stopping, sliding against the surface of the disk, floating in the air, sliding against the surface of the disk and stopping.
It is considered desirable during reading and recording operations to maintain each transducing head as close to its associated recording surface as possible, i.e., to minimize the flying height of the head. This objective becomes particularly significant as the areal recording density increases. The areal density (Mbits/in.sup.2) is the recording density per unit area and is equal to the track density (TPI) in terms of tracks per inch times (.times.) the linear density (BPI) in terms of bits per inch. Thus, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducing head, thereby permitting the head and the disk to be positioned in closer proximity with an attendant increase in predictability and consistent behavior of the air bearing supporting the head. However, another factor operates against that objective. If the head surface and recording surface are too flat, the precision match of these surfaces gives rise to excessive stiction and friction during the start up and stopping phases, thereby causing wear to the head and recording surfaces eventually leading to what is referred to as a "head crash." Thus, there are competing goals of reduced head/disk friction and minimum transducer flying height.
In order to satisfy these competing objectives, the recording surfaces of magnetic disks are conventionally provided with a roughened surface to reduce the head/disk friction by techniques referred to as "texturing." Conventional texturing techniques involve polishing the surface of a disk substrate to provide a texture thereon prior to subsequent deposition of coatings, such as an underlayer, magnetic layer, carbon overcoat and lubricant topcoat, wherein the textured surface on the substrate is reproduced on the surface of the magnetic disk.
A typical magnetic recording medium is depicted in FIG. 1 and comprises a substrate 10, typically an aluminum (Al)-base alloy, such as an aluminum-magnesium (Al--Mg) alloy, chemically plated with a layer of amorphous nickel-phosphorous (NiP). Substrate 10 typically contains sequentially deposited thereon a chromium (Cr) underlayer 11, a cobalt (Co)-base alloy magnetic layer 12, a protective carbon overcoat 13 and a lubricant topcoat 14. Cr underlayer 11, Co-base alloy magnetic layer 12 and protective carbon overcoat 13 are typically deposited by sputtering techniques. Conventional Al-alloy substrates are provided with a NiP chemical plating, typically at a thickness greater than about 10,000 .ANG., primarily to increase the hardness of the Al substrate, serving as a suitable surface for polishing to provide the requisite surface roughness or texture, which is substantially reproduced on the disk surface.
In addition, increasingly high density and large-capacity magnetic disks require increasingly small flying heights, i.e., the distance by which the head floats above the surface of the disk in the CSS drive. The requirement to further reduce the flying height of the head imposed by increasingly higher recording density and capacity render it particularly difficult to satisfy the requirements for controlled texturing to avoid head crash.
Conventional techniques for providing a disk substrate with a textured surface comprise a mechanical operation, such as polishing. See, for example, Nakamura et al., U.S. Pat. No. 5,202,810. Conventional mechanical texturing techniques are attendant with numerous disadvantages. For example, it is extremely difficult to provide a clean textured surface due to debris formed by mechanical abrasions. Moreover, the surface inevitably becomes scratched during mechanical operations, which contributes to poor glide characteristics and higher defects. In addition, various desirable substrates are difficult to process by mechanical texturing. This undesirably limiting facet of mechanical texturing, virtually excludes the use of many inexpensive substrates as well as conductive graphite substrates which facilitate achieving high coercivities.
Alternative texturing techniques to mechanical processing have been attempted. One such alternative to mechanical texturing involves the use of lasers. See, for example, Ranjan et al., U.S. Pat. No. 5,062,021. Another alternative to mechanical texturing is disclosed by Lal et al., U.S. Pat. No. 5,166,006, and involves chemical etching. Such alternative techniques have proven less than successful, in that it is extremely difficult to provide repeatable and controllable textured patterns on non-metallic substrates, such as glass, glass-ceramic materials and electrically conductive graphites. In addition, laser textured substrates also require cleaning.
In copending U.S. Patent Application Ser. No. 08/608,072 filed on Feb. 28, 1996 now U.S. Pat. No. 5,718,811, issued Feb. 17, 1998, a magnetic recording medium is disclosed which has a textured surface formed by sputtering a metallic layer, such as titanium or a titanium alloy, on a non-magnetic substrate, inclusive of a glass, glass-ceramics materials and NiP chemically plated Ni--Mg alloy substrates. It has, however, been found difficult to produce a magnetic recording medium having a suitably high coercivity greater than 2500 Oersteads, such as greater than 3000 Oersteads, particularly greater than 3300 Oersteads, with a sputtered textured layer. In addition, since the topography of the sputtered layer is greatly dependent upon the underlying layer, e.g., substrate, on which it is deposited, process parameters must be optimized for each different type of underlying material, thereby decreasing production throughput. Without such optimization of process parameters, consistently reproducible results are difficult to achieve.
The requirements for high areal recording density impose increasingly greater requirements on thin film magnetic recording media in terms of coercivity, remanent squareness, low medium noise and narrow track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements, particularly a high density magnetic rigid disk medium for longitudinal recording.
The linear recording density can be increased by increasing the coercivity of the magnetic recording medium. However, this objective can only be accomplished by decreasing the medium noise, as by maintaining very fine magnetically noncoupled grains. Medium noise is a dominant factor restricting increased recording density of high density magnetic hard disk drives. Medium noise in thin films is attributed primarily to inhomogeneous grain size and intergranular exchange coupling. Therefore, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
It is recognized that the relevant magnetic properties, such as coercivity (Hc), magnetic remanence (Mr) and coercive squareness (S*), which are critical to the performance of a Co base alloy magnetic thin film, depend primarily on the microstructure of the magnetic layer which, in turn, is influenced by the underlayer on which it is deposited. Conventional underlayers include Cr, molybdenum (Mo), tungsten (W), titanium (Ti), chromium-vanadium (CrV) as well as Cr alloyed with various substitutional elements. It is recognized that underlayers having a fine grain structure are highly desirable, particularly for growing fine grains of hexagonal close packed (HCP) Co deposited thereon.
It has been reported that nickel-aluminum (NiAl) films exhibit a grain size which is smaller than similarly deposited Cr films which are the underlayer of choice in producing conventional magnetic recording media. Li-Lien Lee et al., "NiAl Underlayers For CoCrTa Magnetic Thin Films", IEEE Transactions on Magnetics, Vol. 30, No. 6, pp. 3951-3953, 1994.
Accordingly, NiAl thin films are potential candidates as underlayers for magnetic recording media for high density longitudinal magnetic recording. Such a magnetic recording medium is schematically depicted in FIG. 2 and comprises substrate 20, NiAl underlayer 21 and cobalt alloy magnetic layer 22. However, it was found that the coercivity of a magnetic recording medium comprising an NiAl underlayer, such as that depicted in the FIG. 2, is too low for high density recording, e.g. about 2000 Oersteds.
Lee et al. subsequently reported that the coercivity of a magnetic recording medium comprising an NiAl underlayer can be significantly enhanced by depositing a plurality of underlayers containing alternative NiAl and Cr layers rather than a single NiAl underlayer. Li-Lien Lee et al., "Effects of Cr Intermediate Layers on CoCrPt Thin Film Media on NiAl Underlayers," Vol. 31, No. 6, November 1995, pp. 2728-2730. Such a magnetic recording medium comprising an alternative NiAl layer and Cr layer composite structure is schematically illustrated in FIG. 3.
Adverting to FIG. 3, the depicted magnetic recording medium comprises substrate 30 having sequentially formed thereon Cr sub-underlayer 31, NiAl underlayer 32, Cr intermediate layer 33, and Co alloy magnetic layer 34. It was found, however, that such a magnetic recording medium is characterized by an underlayer structure exhibiting a (110) -dominant crystallographic orientation which does not induce the preferred (1120)-dominant crystallographic orientation in the subsequently deposited Co alloy magnetic layer and is believed to contribute to increased media noise. Li-Lien Lee et al. were able to obtain an underlayer exhibiting a (200)-dominant crystallographic orientation by initially depositing a Cr sub-underlayer directly on the non-magnetic substrate at an undesirably high temperature of about 260.degree. C. using radio frequency (RF) sputtering. However, deposition of a Cr sub-underlayer at such an elevated temperature undesirably results in significantly larger grains than grains resulting from deposition at lower temperatures, e.g. approximating room temperature (25.degree. C.). The formation of larger grains is inconsistent with the very reason for employing NiAl as an underlayer. On the other hand, it is very difficult to obtain Cr (200) crystallographic orientation, even at elevated temperature such as 260.degree. C., on glass and glass ceramic substrates using direct current (DC) magnetron sputtering, which is widely employed in the magnetic recording media industry.
Li-Lien Lee et al. recognized the undesirability of resorting to high deposition temperatures to obtain a (200)-dominant crystallographic orientation in the underlayer structure. It was subsequently reported that an underlayer structure exhibiting a (200)-dominant crystallographic orientation was obtained by depositing a magnesium oxide (MgO) seed layer using radio frequency (RF) sputtering. Li-Lien Lee et al., "Seed layer induced (002) crystallographic texture in NiAl underlayers," J. Appl. Phys. 79 (8), Apr. 15, 1996, pp. 4902-4904; and David E. Laughlin et al., "The Control and Characterization of the Crystallographic Texture of the Longitudinal Thin Film Recording Media," IEEE Transactions on Magnetics, Vol. 32, No. 5, September 1996, pp. 3632-3637. Such a magnetic recording medium comprising a MgO seed layer and NiAl underlayer is schematically illustrated in FIG. 4 and comprises MgO seed layer 41 deposited on substrate 40, NiAl underlayer 42 deposited on MgO seed layer 41, and cobalt alloy magnetic layer 43 deposited on NiAl underlayer 42. Such a magnetic recording medium, however is not commercially viable from an economic standpoint, because sputtering systems in place throughout the industry making magnetic recording media with the conventional structure of magnetic layers epitaxially formed on underlayers are based upon direct current (DC) sputtering. Accordingly, RF sputtering an MgO seed layer is not economically viable.
On the other hand, the objective of having a (200) crystallographic orientation in the underlayers is to induce (1120) crystallographic orientation in the Co alloy layers. Even through media comprising an MgO seed layer and NiAl underlayer has (200) crystallographic orientation in the underlayer, it does not have a dominant (1120) crystallographic orientation in the Co alloy layer, according to Laughlin et al., "The Control and Characterization of the Crystallographic Texture of the Longitudinal Thin Film Recording Media," IEEE Transaction on Magnetics, Vol. 32, No. 5, September 1996, p. 3634. Laughlin et al. reported that the grain-to-grain epitaxial relationship between the (002) NiAl and the CoCrPt film is found to be [1011] CoCrPt//[001] NiAl, and (1210) CoCrPt//(100) NiAl, or (1210) CoCrPt//(010) NiAl. In other words, Laughlin et al. reported that there is no (1120) CoCrPt//(200) NiAl epitaxial relationship found in the films with MgO seed layers and NiAl underlayers. Rather, (200) orientation is identical with (002) orientation. When an FeAl underlayer is used instead of NiAl, it was reported that the (200) FeAl underlayer can only induce a weak (1120) textured CoCrPt by employing a MgO seed layer or a (200) textured Cr seed layer. Li-Lien Lee et al., "FeAl underlayers for CoCrPt thin film longitudinal media," CC-01, 41st Annual Conference on Magnetism and Magnetic Materials, Atlanta, Ga., Nov. 12-15, 1996.
Co-pending application Ser. No. 08/699,759, filed on Aug. 20, 1996 now U.S. Pat. No. 5,866,227, issued Feb. 12, 1999, discloses that Cr films deposited on surface oxidized NiP layers experience smaller grains than Cr films deposited on non-oxidized NiP layers. Co-pending application Ser. No. 08/586,529, filed on Jan. 16, 1996, discloses a method for depositing Cr films on surface oxidized NiP films, wherein the deposited Cr films exhibit a (200)-dominant crystallographic orientation.
In copending Application Ser. No. 08/945,084 filed on Oct. 17, 1997 a magnetic recording medium having high coercivity is disclosed, which magnetic recording medium comprises an seed layer having an oxidized surface formed on a non-magnetic substrate, a chromium-containing sub-underlayer on the oxidized surface of the seed layer, a nickel-aluminum or iron-aluminum underlayer, a chromium-containing intermediate layer on the underlayer and a magnetic layer on the intermediate layer.
There exists a need for magnetic recording media with repeatable and controllable sputter textured surface patterns exhibiting improved floating and improved sliding-wear-resistant characteristics, low noise and high coercivity.